1. Field of the Invention
The present invention relates to a method for the encapsulation of a molded member of ceramic material, particularly a silicon ceramic for high-temperature isostatic pressing.
During hot or high-temperature isostatic pressing (HIP) porous molded members of silicon ceramic are compressed through the simultaneous application of high pressure on all sides thereof and of high temperature. By means of the high-temperature isostatic pressing there can be improved the mechanical strength at elevated temperature as well as resistance to oxidation through a reduction in the porosity of the molded member. The pressures employed during high-temperature isostatic pressing (HIP) lie in an order of magnitude of 3000 bar, with the temperature being about 1750.degree. C. Normally, gas is utilized as a pressure transmitting medium. In order to prevent this gas from penetrating into the pores of the molded member, the molded member must be encapsulated in a pressure-tight manner.
2. Discussion of the Prior Art
Heretofore, this encapsulation has been effected in that the molded ceramic member is enclosed in an evacuated glass container, wherein the glass container becomes viscous during the high-temperature isostatic pressing and adheres to the molded ceramic member as a glass coating or layer.
In this known method of encapsulating the molded ceramic member there exists the danger at a too low viscosity of the glass container melting during the high-temperature isostatic pressing, the glass coating will penetrate into the pores of the molded member, and that there will occur a chemical reaction between the glass and the silicon ceramic of the molded member. This may impair the strength of the surface region of the molded member to an inadmissible extent. Furthermore, due to the excessive penetration of the highly fluid glass into the porosity of the molded member there may be produced such an intimate bond between the glass envelope and the molded member that upon the removal of the glass envelope after the high-temperature isostatic pressing, it may not be possible to avoid damaging of the molded ceramic members, particularly if they are of complicated, thin-walled configurations. On the other hand, there is also present the danger that at too high a viscosity of the glass container melting during the high-temperature isostatic pressing process and of the resultant glass coating adhering to the molded ceramic member, there can occur absolutely undesirable deformation of the molded ceramic member.